On Sunday 09 September 2012 18:10, Roland Dreier wrote:
>
> Please look at commit 7cf9c2c76c1a ("[PATCH] radix-tree: RCU lockless
> readside")
>
Roland,
What about the following note (from the commit diff mentioned above):
+/**
+ * Radix-tree synchronization
+ *
+ * The radix-tree API requires that users provide all synchronisation (with
+ * specific exceptions, noted below).
+ *
+ * Synchronization of access to the data items being stored in the tree, and
+ * management of their lifetimes must be completely managed by API users.
+ *
+ * For API usage, in general,
+ * - any function _modifying_ the the tree or tags (inserting or deleting
+ * items, setting or clearing tags must exclude other modifications, and
+ * exclude any functions reading the tree.
+ * - any function _reading_ the the tree or tags (looking up items or tags,
+ * gang lookups) must exclude modifications to the tree, but may occur
+ * concurrently with other readers.
+ *
+ * The notable exceptions to this rule are the following functions:
+ * radix_tree_lookup
+ * radix_tree_tag_get
+ * radix_tree_gang_lookup
+ * radix_tree_gang_lookup_tag
+ * radix_tree_tagged
+ *
======= JPM -- Note the following text! =========================
+ * The first 4 functions are able to be called locklessly, using RCU. The
+ * caller must ensure calls to these functions are made within rcu_read_lock()
+ * regions.
===========================================
+ * Other readers (lock-free or otherwise) and modifications may be
+ * running concurrently.
Roland, the above states explicitly that radix_tree_lookup should be called
only under rcu_read_lock()!
This is NOT the case in our driver, in mlx4_eq_int()/mlx4_cq_completion() .
Looks to me, then, that the spinlock in mlx4_cq_completion() is required.
- Jack
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